1. Field of the Invention
The present invention relates to a method for connecting battery cells in a battery, to a battery, to a corresponding monitoring device and to a corresponding computer program product.
2. Description of the Related Art
Normally, the state of charge (SOC) of a battery is determined by measuring the open circuit voltage at the entire battery or at individual cells. If at a particular time, a measurement is possible only under load, then the open circuit voltage may be calculated based on the measured voltage, the current flow and the resistance. This calculation may be corrected later, once a comparison with the measurement of the open circuit voltage (without load) is possible.
In addition, the current flow into or out of the battery may be integrated, in order to obtain an indication of the state of charge. However, since errors in current measurement resulting from integration continually add up, it is always necessary to compare this methodology with the state of charge calculated on the basis of the open circuit voltage.
The extremely important correlation between open circuit voltage and state of charge is only possible, however, if the open circuit voltage (OCV) characteristic curve as a function of the state of charge (SOC) exhibits a sufficient gradient, i.e., d(UOCV)/d(SOC)≠0. Some important electrochemical systems, however, have a very flat voltage characteristic curve, for example, LiFePO4 and also lithium-sulfur (2 Li+S—>Li2S). So far, it has been very difficult to use these important systems in batteries, the state of charge of which must be estimated as precisely as possible, such as, for example, in a plug-in hybrid vehicle and an electric vehicle.
The aforementioned problems of determining the state of charge may, particularly near the end of a discharge, lead to a residual capacity being displayed in the battery management system, whereas the actual capacity of the battery is already fully spent. The driver of an electric vehicle would be stuck, even though the state of charge indicator in the car has displayed a remaining capacity.
Where the aforementioned cell chemistry systems are concerned, this problem may mean that such systems cannot be used in electric vehicles or in similar applications.
Published German patent application document DE 10 2010 002 326 A1 describes a method for balancing states of charge of a battery having multiple battery cells and a corresponding battery management system and a battery.